wiggle the body of the sub until you break the screw. the screw will break way before you damage the basket of the sub. then, when youre ready to re-install, just screw another screw right in there and it will push the old one into your box. then take that screw and the remains of the broken one out of your box and place the sub in the hole. proceed
if you don't want to do all that unscrew all the other screws so all the weight is on that one screw. then it'll be able to wiggle and just wiggle it to the point that its loose enough to drill out